This invention relates to a video processing system including a video output amplifier for providing video drive signals to an image display device such as a kinescope. In particular, this invention concerns a video output driver amplifier which can automatically self-regulate the peak beam current output provided to a display.
In a typical video processing system, video output amplifiers are used to provide amplified video signals to intensity control electrodes (e.g., cathodes) of a display device (e.g., kinescope). The amplified video signals are provided in the form of, for example, R, G, and B beam currents to the respective cathode of an image display device.
It is desirable to be able to limit the beam current provided to the display at a predetermined level. One reason is that if an excessive output beam current is developed in response to a large amplitude video signal representative of highly saturated color image information or peak white image information at the input of the video amplifiers, the output transistors of the amplifiers may saturate in response thereof.
A saturated output amplifier can produce an objectionable white or colored image smear following a displayed image area produced by a large amplitude video drive signal. For example, if a white image area is displayed and the biasing of the red video output transistor is such that the red output transistor saturates, a red color smear following the white image is likely to be produced as a result of the red video output transistor exhibiting a saturated conductive state together with the slow xe2x80x9crecovery timexe2x80x9d of the red output transistor due to transistor charge storage effects.
In addition to causing a receiver to produce a degraded image, excessive beam current may also cause degradation of the performance of the receiver""s deflection system, electron beam spot defocusing and picture blooming. Such a high beam current may also exceed the safe operating capability of the kinescope, possibly causing damage to the kinescope and associated circuit components.
On the other Hand, it is desirable to be able to provide the maximum current allowable to the kinescope so that the picture produced by the display can be as bright and have as much contrast as possible. This is particularly desirable in a projection-type display where the pictures on the projection display tend to be dimmer than those on a direct view tube.
Various beam current limiting systems are known. One common approach to limiting beam current to a CRT is shown in FIG. 1. This approach involves monitoring the average beam current at the high voltage supply of a CRT 101 and providing a control signal representative of the magnitude of the average beam current. The average beam current is derived from a filtering capacitor C1. If the average beam current exceeds a predetermined threshold, then the parameters of the contrast and brightness adjustment stages 104A and 104B in the video processing circuit 104 are changed in accordance to the control signal. By reducing the contrast and brightness of the video signal at the input of the video driver stage, the beam current at the output of the video driver amplifier (20xe2x80x2, 22xe2x80x2, 24xe2x80x2) is also reduced. The beam current is therefore controlled via the contrast and brightness processing stages of the video processing system, long before the video signal reaches the video driver stage.
EP 0 680 226 A2 discloses a kinescope driver apparatus having a gamma correction feature. Gamma correction is desirable to compensate for the non-linear relationship between the input signal and the light output of a picture tube. The apparatus comprises buffer amplifier 70 coupled to cathode K1 via resistor 79, which senses the cathode current. Buffer amplifier 70 is coupled to linear amplifier 60 via network 80. The feedback signal provided by network 80 provides gamma correction as shown in FIG. 1. However, the apparatus does not provide a peak beam limiting feature which limits the output to the cathode to a predetermined level when the cathode current exceeds a predetermined peak cathode current level.
U.S. Pat. No. 4,599,642 discloses a video signal processor that selectively couples a beam current control signal to a luminance processor and a chrominance processor during white level drive control intervals and black level bias control intervals. The processor includes a beam limiter control circuit that receives a summed signal representative of a combination of cathode currents and provides a control signal to the luminance and chrominance processors. Here, the apparatus uses a combined cathode current signal to generate the control signal and provides the control signal to various processors, rather than to a particular drive amplifier associated with a selected cathode.
The present inventor recognizes that, however, it is important to monitor and control not only the average beam current but also the peak beam current. Excessive average beam current and peak beam current can cause different problems mentioned above. An average beam current limiter is needed particularly to protect the high voltage supply system from excessive power drain, and the display from excessive power dissipation. Excessive average beam current may also result in shadow mask beam location error, therefore causing color error, in direct view tubes with a shadow mask.
A peak beam current limiter, on the other hand, is needed to prevent excessive instantaneous beam current that may cause excessive aging of the phosphor screen and poor spot size. Poor or large spot size is equivalent to poor focus and loss of resolution.
Furthermore, it is desirable to have both types of beam current limiters in the same video processing system because the relationship between peak and average level of a video signal cannot be predicted. For example, white characters in a black background can result in very high instantaneous peak beam current, yet low average beam current, if a television set is a high performance set, with significant drive capability or gain.
In accordance with the present invention, a video signal processing system including an image reproducing device for displaying video information in response to a video signal applied thereto is provided. The processing system includes an apparatus comprising a video output driver stage with a video signal input and a video signal output for providing an amplified video signal. The apparatus is characterized in that the video output driver stage comprises means for amplifying the input video signal and coupling said output signal to the image reproducing device. The output driver also includes a sensing output for providing thereat a sensed signal representative of current conducted by the image reproducing display device. The video driver stage further comprises measurement and control means for sensing the sensed signal and for providing a feedback signal to an input component of said video output driver stage for limiting the peak beam current conducted by said image reproducing display device to a predetermined value.